It is common practice for an electric motor to use lubricant oil in order to provide a smooth rotational drive. Causing the lubricant oil to function as cooling oil by circulating the same through an oil flow path extending around the electric motor, the electric motor may operate efficiently by cooling heat generated during the rotational drive. A heat exchanger may be disposed in the oil flow path on the way or different cooling oil may be circulated. For example, JP-A 2006-254616 discloses a cooling system by circulating cooling oil. This oil circulation system for electric motors has implemented a pump start-up procedure to definitely start an electric oil pump by repeating start-up operation more than once in the event that the electric oil pump fails to start circulating cooling oil for some reason.
However, the oil circulation system for electric motors disclosed by JP-A 2006-254616 poses a problem that its cooling function will not work unless the start-up of the electric oil pump succeeds because the cooling oil cannot be made to circulate until the start-up of the electric oil pump succeeds. If the oil circulation system for electric motors disclosed by JP-A 2006-254616 were applied to circulation of lubricant oil for the electric motors, lubrication property might become insufficient due to the shortage of lubricant oil supply to sliding parts of the electric motors.